I don't expect this wine to be dead, but I wouldn't suggest holding onto it any longer. Sebatiani does a lot of resturant business, and consequently, makes their wines drinkable early on, rather than making them in an ageing friendly style.

Larger bottles age slower, so you magnum should still be fine, but I'd recommend popping that cork sooner than later.

And I will say, in the realm of cheaper cali merlot, Sebatiani makes a very elegant wine for the price. Enjoy!

If it has a nice nose (aroma) and taste, then it's okay. If it smells moldy or tastes flat (no fruit left) or has turned more brown than red, those are signs it may be over-the-hill or oxidized. (A wet cardboard smell may indicate that the cork has failed or that TCA is present).